1. Field of the Invention
This invention generally relates to a veneer lathe and, more particularly, to an apparatus and method for applying supplemental torque to the outer periphery of a log.
2. Prior Art
Veneer is the most valuable product recovered from wood and, as the price of stumpage rises, the maximum recovery of veneer has become more important. Veneer lathes of the type concerned here axially support a veneer bolt or log utilizing lathe chucks which engage the ends thereof. As the log is rotated about its longitudinal axis, a tangential knife and pressure bar are advanced radially towards the center of the log to cut off a thickness of wood which passes between the cutting edge of the knife and the pressure bar. Generally, the larger the bolt diameter, the greater the torque required for a given cutting force since T=FL where T is torque, F is cutting force and L is the distance between the line of action and the axis about which the wood bolt is turned. Known veneer lathes of the type described above possess certain definite disadvantages, especially with respect to veneer bolt spin-out.
Veneer bolt spin-out occurs when the torque required for cutting exceeds the torque that can be delivered by the chucks that grip the bolt ends. Spin-out can occur at any time during the peeling process; however, it is most likely to occur when the required torque is highest such as at the beginning of the cutting operation when the bolt has its largest diameter or after the transition from outer to inner chucks, since the ability of the inner chucks to transmit torque is significantly less than that of the outer chucks. Since the end result of veneer bolt spin-out is the loss of the bolt as a source of veneer, a solution to this problem would eliminate the significant loss in time and equipment usage already invested in a log that has spun-out.
Because the torque is delivered to the log through the chucks, the logical solution to the problem would be to increase the amount of torque deliverable by optimizing the chuck design. Although the chuck design can be optimized, veneer bolt spin-out remains a problem especially with increased emphasis on peeling to smaller diameter cores, since the diameter of the average bolt has diminished. Further, the short supply of logs is now so serious that a poor quality log, having a softer core, must be used. Therefore, when the strength of the wood is inadequate to provide the required torque necessary to peel the bolt, even an optimally designed chuck will spin-out. Realizing that optimized chuck design cannot provide the ultimate solution to a veneer bolt spin-out, alternative means for providing supplemental torque to the log have been developed.
One approach for providing supplemental torque is through a power back-up roll which engages the peripheral, outer surface of the bolt to apply a rotational force to the bolt. Prior art devices, such as U.S. Pat. No. 2,884,996, are known having power back-up rollers which are installed on existing lathes for bracing the bolt to counteract the resultant force set up by the advancing knife and pressure bar which tends to bend a bolt of relatively small diameter away from the knife. While some existing back-up rolls provide this reaction while maintaining a constant pressure or load on the bolt, the most effective back-up rolls are actually position-controlled rather than load-controlled. In contradistinction, an auxiliary torque back-up roll primarily designed to supply supplemental torque to a bolt requires maximizing the friction and normal force between the bolt and the roll to maximize the transmitted power from the back-up roll to the log. Since the magnitude of the frictional force, and thus the torque that can be transmitted, is proportional to the product of the coefficient of friction and the normal force; it is important to maintain static friction between the power roller and the log surface as well as controlling the normal force between the power roll and the log.